PREVIOUS INTERPRETATIONS OF CNIDARIA 111 



The change in symmetry of the form from a bilateral to a radial 

 symmetry, and simultaneously to the general polyp form of the 

 entire body, is connected with this. On the other hand, we 

 can find progressive morphogenetic processes in individual 

 organs. Here we have first to mention the progressive deve- 

 lopment of the cnidae together with the tentacles; the strong 

 extensibility connected w4th an increased contractibiUty (a spe- 

 cial development of the cutaneous muscle tube), connected 

 with the ability to move in all directions (nutations). We must 

 certainly attribute the evolution of secondary freely swimming 

 medusae, and, finally, the liberation of whole cormi (Siphono- 

 phora) and the formation of a special impersonal floating appa- 

 ratus, the pneumatophore (cf. Had^i, 1954), to such progressive 

 properties. 



Both regressive as well as progressive developments have 

 reached their climax in the Hydrozoa which can be easily un- 

 derstood if viewed from our standpoint. So far it has been the 

 regressive element in particular which has been taken into 

 consideration, and it has been wrongly interpreted as a sign 

 of a primary simplicity. This has been especially done 

 because of the epithelization of the three body layers which 

 is a characteristic of all the sedentary groups of Metazoa; 

 the intermediate body layer can be strongly reduced by way of 

 such an epithelization so that locally it can almost completely 

 disappear. It was in this way that T. H. Huxley concluded 

 (1875) with regard to medusae, and Allman with regard to 

 Hydroidea, that these animals consisted of two body layers 

 only, an ectoderm and an entoderm. This circumstance, to- 

 gether with events that take place during their ontogenies, 

 was used by Haeckel when he formulated his gastraea theory. 

 As a matter of fact, however, there is not a single species in 

 Cnidaria which would really consist of two "skins" or epi- 

 thelia only. If we disregard sex cells which cannot be properly 

 attributed to any body layer even in those cases where they 

 adhere closely either to the ectoderm or to the entoderm, 

 there have always been found other cells between these two 



